1. Field of the Invention
This invention relates to a method for the production of an organic fluorine compound. More particularly, it relates to an improvement in a method for the production of an organic fluorine compound by a so-called halogenation exchange reaction, i.e. the reaction of an organic chlorine or bromine compound with a fluorinating agent in benzonitrile as a solvent.
2. Description of the Related Art
The so-called halogen exchange reaction which comprises exchanging a halogen atom for a fluorine atom by the action of an alkali fluoride on an aromatic halide has been known for a long time. Generally as the solvent for this reaction, such so-called aprotic polar solvents as dimethyl sulfoxide (DMSO), sulfolane (TMSO2), N-dimethyl formamide (DMF), N-methyl-2-pyrrolidone (NMP), and dimethyl sulfone (DMSO2) have been mainly used. The halogenation exchange reaction, therefore, is carried out at a temperature not higher than the boiling point of the relevant solvent [Ishikawa, Journal of Organic Synthetic Chemical Society, Vol. 25, page 808 (1967), M. Hudlicky, Chemistry of Organic Fluorine Compounds, page 112 (1976), John Wiley and Sons Press, etc. refer, for example]. Cases of the reaction incorporating such an interphase transfer catalyst as a crown compound in the reaction system for the purpose of accelerating the reaction velocity have been reported.
When the halogen exchange reaction is performed by the method described above, however, the halogen of an aromatic monohalide and an aromatic polyhalide having a halogen substituent at the ortho or para position of the electron attractive group (such as, for example, xe2x80x94CN and xe2x80x94NO2) easily succumbs to halogen exchange but the halogen at the meta position does not easily succumb to halogen exchange.
The solvents in popular use, when heated to an elevated temperature or kept heated for a protracted duration for the purpose of improving yields, undergo a decomposition reaction or induce a secondary reaction with a raw material or with the product of reaction and eventually fail to improve the yield. Further, in the recovery or reclamation of the solvents after use, these solvents are at a disadvantage in not allowing easy commercial use of recovered or reclaimed solvents. With a view to overcoming the drawback of these solvents being unusable at elevated temperatures, the method which carries out the reaction in an autoclave at a high temperature in the range of 200xc2x0-500xc2x0 C. without use of a solvent has found popular acceptance. The case of effecting halogenation exchange of tetrachlorophthalonitrile for tetrafluoroterephthalonitrile in an autoclave at a temperature of 300xc2x0 C. without use of a solvent has been reported, for example, in Ueda et al., Bull. Chem. Soc. Japan, Vol. 40, page 638. Since this method uses no solvent, the reaction which is exothermic in form allows no easy temperature control and the reaction, when completed, suffers a large volume of carbide to persist as a fast deposited residue on the inner wall of the reaction vessel. Thus, the method may well be regarded as a commercially impracticable approach.
As a means to solve this problem, we have disclosed in U.S. Pat. No. 4,684,734 a method for producing an organic fluorine compound by causing an organic chlorine or bromine compound to react with a fluorinating agent in benzonitrile as a solvent at a temperature in the range of 190xc2x0-400xc2x0 C. under at least the spontaneously generated pressure. This method, while converting the organic chlorine or bromine compound through halogen exchange into a corresponding organic fluorine compound, enjoys thermal stability even at an elevated temperature because benzonitrile as a solvent is thermally stable and further avoids inducing such a secondary reaction with a raw material or the product of reaction as observed in the case of other solvents. Thus, the solvent ensures safety use at high temperatures in the range of 190xc2x0-400xc2x0 C., allows the reaction temperature to be elevated as required, and contributes to the improvement of yields. It is also at an advantage in avoiding such a secondary reaction with a raw material or the product of reaction as encountered by other solvents. Owing to the use of this solvent, this method, unlike the method using no solvent, is at an advantage in allowing easy control of temperature and precluding the formation of a large amount of carbide. This method, when commercially used, has the advantage of forming the product aimed at in high yields.
This method of production, however, still has such problems as generating during the halogen exchange reaction acid fluorides of an aromatic compound in an amount in the approximate range of 1000-8000 ppm (as fluorine ion to be generated) based on the amount of the organic fluorine compound and exposing the plant facilities, during the halogen exchange reaction and at the subsequent step of production, to corrosion owing to the formation of hydrofluoric acid (fluorine ions), a corrosive substance originating in the acid fluorides of the relevant aromatic compound.
An object of this invention, therefore, is to provide a novel method for the production of an organic fluorine compound.
Another object of this invention is to provide a method for producing an organic fluorine compound by causing an organic chlorine or bromine compound to react with a fluorinating agent in benzonitrile as a solvent, which method is capable of preventing the occurrence of a benzoic acid fluoride during the halogenation exchange reaction.
Still another object of this invention is to provide a method for producing an organic fluorine compound by causing an organic chlorine or bromine compound to react with a fluorinating agent in benzonitrile as a solvent, which method is capable of removing the benzoic acid fluoride generated during the halogenation exchange reaction.
We have made a diligent study in search of a novel method for producing an organic fluorine compound with a view to accomplishing the objects mentioned above. We have perfected this invention as a result.
Specifically, the objects of this invention mentioned above are accomplished by (1) a method for producing an organic fluorine compound which comprises causing an organic chlorine or bromine compound to react with a fluorinating agent in benzonitrile as a solvent in the presence of the oxide, hydroxide, and/or carbonate of an alkali metal or alkaline earth metal under preventing the occurrence of acid fluorides of an aromatic compound or effecting removal of acid fluorides of the aromatic compound formed.
The objects of this invention mentioned above are further accomplished by (2) a method for the production of an organic fluorine compound, which comprises causing an organic chlorine or bromine compound to react with a fluorinating agent in benzonitrile as a solvent, treating the reaction solution or reaction product with an aqueous liquid, and then separating a water layer thereby removing acid fluorides of an aromatic compound.
The objects of this invention mentioned above are further accomplished by (3) a method set forth in (2) above, where in the aqueous liquid is an acidic aqueous liquid.
The objects of this invention mentioned above are further accomplished by (4) a method set forth in (2) or (3) above, wherein the reaction is carried out in the presence of oxide, hydroxide, and/or carbonate of an alkali metal or alkaline earth metal.
The objects of this invention mentioned above are further accomplished by (5) a method set forth in any of (1)-(4) above, wherein the organic chlorine or bromine compound is a chloride or bromide aromatic compound.
The objects of this invention mentioned above are further accomplished by (6) a method set forth in any of (1)-(5) above, wherein the fluorinating agent is at least one member selected from the group consisting of fluorides of alkali metal and alkaline earth metals.
The methods of this invention mentioned above are further accomplished by (7) a method set forth in any of (1)-(6) above, wherein the acid fluoride of an aromatic compound is benzoic acid fluoride.
The objects of this invention mentioned above are further accomplished by (8) a method set forth in any of (1)-(7) above, wherein the organic chlorine or bromine compound is pentachlorobenzonitrile and the organic fluorine compound is pentafluorobenzonitrile.
The objects of this invention mentioned above are further accomplished by (9) a method for the production of pentafluorobenzoic acid, which comprises causing pentachlorobenzonitrile to react with a fluorinating agent in benzonitrile as a solvent thereby obtaining pentafluorobenzonitrile, hydrolyzing the benzoic acid fluoride present in the resultant reaction solution with an acidic aqueous liquid and inducing separation of the hydrolyzate on the water layer side, and then hydrolyzing the pentafluorobenzonitrile resulting from the treatment in an aqueous sulfuric acid solution thereby producing the pentafluorobenzoic acid.
By this invention, the corrosion possibly caused during the process of halogen exchange reaction can be prevented.
This invention can further prevent the occurrence of fluorine ions, a corrosive substance, during the process of hydrolysis which follow the process of halogen exchange reaction.